Coxiella burnetii, the rickettsial agent of Q fever in humans, is an obligate intracellular bacterium. Occasionally, acute disease is followed by life-threatening chronic endocarditis for which there is no consistently effective treatment. The long-term goal of this research is to discover the biochemical/molecular mechanisms that account for survival of C burnetii in the phagolysosomes of phagocytic cells and other host cells. Preliminary studies have revealed that the organism possesses significant acid phosphatase activity and that it may be responsible for inhibiting the oxidative burst in human neutrophils; this indicates that the enzyme may be a major virulence determinant. Phosphatases have been shown to occur at or near the surface of Leishmania promastigotes and Legionella micdadei, a bacterium that is phylogenetically related to C burnetii. The phosphatases inhibit the oxidative burst and concomitant production of toxic superoxide anion by phagocytosing human neutrophils; this probably accounts, in large part, for the survival of these parasites within phagocytes. The specific aims of the proposed research are to: (i) characterize the biochemical and biophysical properties of the recently discovered periplasmic C burnetii acid phosphatase; (ii) clone and characterize the acid phosphatase gene using available gene banks; (iii) examine the effect of the enzyme on human neutrophils. Preliminary evidence strongly indicates that partially purified C burnetii acid phosphatase inhibits the oxidative burst of human neutrophils; this will be verified with purified phosphatase. The site and mechanism of action of the enzyme will be investigated. Whether or not the phosphatase is released from the parasite during the infection process will be determined. These aims will provide basic information about an enzyme which may play a key role in blocking the host cell's capacity to mount a response against the invading and resident parasites. Attainment of these goals will result in a better understanding of the parasite's capacity to circumvent the host's defenses during invasion of and subsequent survival and proliferation within phagocytes and other host cells. Such knowledge is essential for devising strategies for the development of efficacious drugs and vaccines for treating/preventing Q fever, including life- threatening endocarditis.